when tinkering with fuel for thorium using plutonium and thorium you can achieve infinite fuel by using productivity modules on the assembly machine step of the fuel rods. this confuses helmod as well

fuel.... rods?

I wouldn't call it infinite, but you do get more fuel cells out of the assembling machine if you put productivity modules in them. You can get more materials back out by using productivity modules when you recycle the spent cells too. And yes, you do get a lot of Uranium 235 back out of a thorium cell, but since you need to put A LOT of thorium and only a little bit of U235 in the cell, it shouldn't be an issue.

Remember, overhaul turned on is the default, and how it's meant to be played. overhaul off is legacy, and only exists because some people liked the old way. The old way, there was a fuel cell that used Thorium and Plutonium. I can see that this could become infinite with productivity, but again, this is legacy, the option only exists because some people like it better that way.

here is a photo for refernce https://photos.app.goo.gl/WGKBXKjdwNBkPSsM9

here is a photo for refernce https://photos.app.goo.gl/WGKBXKjdwNBkPSsM9

Yeah, that's the old version, with the overhaul turned off.
Turning on the overhaul should fix this issue.

Oh okay how do I do that?

From the main menu... Settings, mod settings, Startup tab, scroll down to the "Bob's Metals, Chemicals and Intermediates mod" section, then turn on the "Nuclear fuel update" option.

thanks it works but it did reset some of my research